1. Field of the Invention
The invention relates to a graphite electrode for arc furnaces with several electrode sections connected by threaded nipples.
2. Description of the Prior Art
Graphite electrodes used in arc furnaces and arc reduction furnaces are as a rule composed of several electrode sections connected to each other (this term is understood in the following to also include electrodes which consist predominantly of carbon and have not been subjected to a graphitizing treatment). The connecting means are predominantly conical, double-conical or cylindrical threaded nipples of the same material. The nipple is screwed into box-like recesses which start from the end faces of the electrode sections and are provided with threads. Since the cross-section of the nipple accounts for only a part of the electrode cross-section it is generally necessary to increase the strength of the nipple beyond that of the adjacent electrode sections. For this purpose, only selected materials are used for making the nipple and the accessible pore volume is decreased by impregnation with pitch or other impregnating media. One detrimental consequence of this method is an increase of the radial thermal coefficient of expansion and of the modulus of elasticity and thereby, a noticeable reduction of the resistance to sudden temperature changes (temperature shock resistance). Like the overall electrode, the screw connections between the electrode sections are subjected to large temperature variations and temperature gradients during the operation of the electrode arc furnaces, particularly in the production of steel. With the customary fast heating, tangential stresses are generated in the box wall, i.e. recess wall due to radial temperature differences and the material difference between the nipple and the electrode section. Also tension stresses occur while the electrode is cooling down, for instance to connect a nipple of a new electrode section, by shrinking the box into the nipple. Thus, both the stresses caused by fast heating and by cooling down can cause the box wall to break. Another widespread defect is the fracture of the nipple itself which in double-conical nipples happens more frequently in the equatorial plane. Causes of the fracture are tensional stresses which are generated by different thermal coefficients of expansion of the nipple and the electrode sections in the longitudinal direction. Since electrode fractures increase the consumption of electrodes and, above all lead to interruptions of the furnace operation, it has been attempted repeatedly to reduce the tension build-up within the electrode and the fracture rate by structural and material changes of the electrode parts and the connections between the parts. It has, for instance, been proposed to limit radial and tangential stresses due to abrupt temperature changes by means of slots or notches which are made in electrode sections and threaded nipples and run substantially parallel to their longitudinal axis (U.S. Pat. Nos. 2,527,294; 2,603,669). According to other proposals, stress concentrations are to be prevented by specially designed screw threads, for instance, by a screw thread with a continuously changing thread depth (German Patent No. 25 55 683). Disadvantages of the proposed practice are the deviation from standard threads and a reduction of the static fracture load of the electrode strand. Among the attempts to solve the fracture problem, it has been proposed to increase the strength of electrode sections and nipples substantially, particularly by working carbon fibers into the material customarily consisting of graded petroleum coke and a pitch binder for producing these products (German No. DE-OS 26 59 374). Carbon fibers are used because of their great strength and stiffness primarily for reinforcing synthetic resins, as well as metallic and ceramic materials. In the compounds used in the manufacture of the electrode sections and nipples, it is extremely difficult, however, to uniformly distribute the carbon fibers, which are added in an amount of about 1%, since in addition, a major part of the relatively brittle fibers breaks in this treatment, the effect obtained is small, i.e. the fracture rate of electrode sections is not reduced appreciably.